Generally, protection films are widely used to protect surfaces of metal products, plastic plates and polarizers for electronic materials. Acrylic pressure sensitive adhesives are widely used as adhesives for such protection films because of transparency and their resistance to corrosion. The acrylic pressure sensitive adhesives currently used are copolymers prepared by copolymerization of a (meth) acrylic acid alkyl ester monomer with a monomer having a functional group such as a carboxyl, hydroxyl, or epoxy group, which are crosslinked with a polyisocyanate compound, a melamine resin, or an epoxy resin. The surface-protection films are provided not only to protect adherends such as polarizing films, plastic plates, electrical/electronic appliances or automobiles from static electricity but also to protect them from contamination by dust, etc. Therefore, the antistatic properties as well as the peel strength beyond a certain level of adhesion are very important characteristics for the surface-protection film.
Generally, static electricity is generated by frictional charging when two different objects are rubbed together, and peel charging when two adhered objects are detached from each other. Such static electricity can result in adsorption of contaminants such as dust, electrostatic breakdown of devices, malfunction of measurement instruments, and fire. Particularly, with increasing demand for liquid crystal displays fueled by the growing market for computers, liquid crystal TVs, multifunctional mobile phones, etc., integrated accessory devices are more vulnerable to static electricity, and thus prevention of static electricity in liquid crystal displays has become a matter of concern. In a process of manufacturing a large liquid crystal display using a large polarizer, peeling off a release film from the polarizer generates excessive static electricity, which affects the orientation of liquid crystals in the liquid crystal display and causes defects in picture images.
In order to prevent the generation of static electricity, an antistatic layer can be formed on an outer surface of the polarizer or a base film of the protection film. However, this is not very effective and does not prevent the generation of static electricity from the beginning. Therefore, in order to fundamentally prevent the generation of static electricity, it is important for the pressure sensitive adhesive to have an antistatic property.
A conventional acrylic pressure sensitive adhesive includes a chelate compound and a metal salt as additives to achieve the antistatic property. However, when a large amount of chelate compound is added, the additives are transferred to an adherend when the pressure sensitive adhesive has been applied.
To be specific, conventional methods for imparting an antistatic property to a pressure sensitive adhesive include: a method of adding conductive metal powder or materials having conductive components such as carbon particles to a resin; and a method of adding surfactant-type ionic or non-ionic materials. However, these methods require a large amount of additive, thereby degrading a pressure sensitive adhesive property by migration of the additive to the surface of the pressure sensitive adhesive.
Japanese Patent Publication No. 1993-140519 discloses a method of preventing generation of static electricity by adding an ethylene oxide-modified phthalic acid dioctyl plasticizer to yield a pressure sensitive adhesive with flexibility. However, this method has the problems of migration to the surface of the polarizer and not eliminating static electricity generated in the first place.
Korean Patent Publication No. 2004-0030919 discloses a method of preventing generation of static electricity by adding 5 wt % or more organic salt. However, this method requires a large amount of expensive organic salt.
Korean Patent Publication No. 2006-0128659 discloses a method of preventing chlorosis under high temperature and high humidity conditions by adding a chelating agent capable of forming a complex with a metal ion, and a metal salt to an acrylic pressure sensitive adhesive. However, this method also has a problem of a decrease in low-rate peel strength due to a large amount of additive.